1. Field of the Invention
This invention relates generally to light emitting diodes known commonly as "LED". More particularly, the present invention relates to a process for making LED's of the type wherein one of paired leads has a cup end with a reflective inner surface for mounting a semiconductor chip to effectively emit light through a transparent or semitransparent resin package.
2. Description of the Prior Art
Conventionally, LED's of the above-mentioned type are typically manufactured by using a leadframe which is punched out from a thin metallic plate, as disclosed in Japanese Patent Application Laid-open No. 59-229884 for example. The leadframe has pairs of leads arranged at constant pitch.
In the manufacturing process utilizing the leadframe, the pairs of leads are cut off at the final stage of the process, and the remaining portion of the leadframe is discarded. Thus, the use of the leadframe inevitably has a problem of material waste, consequently leading to a cost increase.
In an attempt to reduce the production cost, the inventors have previously proposed the use of a metallic wire as a material for LED leads, as disclosed in Japanese Patent Application Laid-open Nos. 3-27540 and 3-50747.
In this known process, specifically, the material wire is first subjected to axial compression at a forward end thereof to provide a cup end, and a portion of the wire including the cup end is cut off to provide a wire segment of a predetermined length. This operation is repeated until the material wire is exhausted, thereby providing a plurality of similar wire segments.
Then, each of the wire segments is bent into an elongate U-shape to have a pair of legs connected together through an integral connecting web. One of the paired legs has the cup end. The respective U-shaped wire segments are thereafter attached to a carrier band for collective transfer and handling.
During transfer, a semiconductor chip is mounted in the cup end of the one leg of each wire segment, and the chip is connected to the other leg through a thin wire. Then, a transparent or semitransparent resin package is formed, by using a suitable mold, to enclose the respective free ends of the paired legs. Finally, the connecting web is cut off from the wire segment.
The above prior art process is advantageous in avoiding material waste because substantially the entire portion of the material wire is used to make leads. However, the prior art process is still disadvantageous in the following respects.
First, the inner surface of the lead cup end is required to work as a reflective mirror surface for effectively emitting light. With the prior art process, however, since the cup end is formed by axially compressing a cut end (forward end) of the material wire, the core wire metal is inevitably exposed at the inner surface of the cup end even if the material wire is previously plated with a glossy metal. Thus, it is necessary to provide an additional step of forming a glossy coating or plating on the inner surface of the cup end, consequently resulting in a production cost increase.
Secondly, when the cup end is formed by axially compressing the material wire endwise, there is a limitation on the design freedom in selecting the size and shape of the cup end. This is partly because of the limited diametrical expansibility of the material wire and partly because of the fact that the wire buckles easily at the time of axial compression unless a portion of the wire subjected to axial compression is kept small.